[R] How to formulate an (effect-modifying) interaction with matching variable in a conditional logistic regression?

an example below using data from Hosmer and Lemeshow's Applied Logistic Regression (2nd ed. p.230 onwards) I can solve the problem by introducing new variables that contains the interaction I want, but I'd like to avoid this unwieldy solution. Can you suggest a better one? Thank you for

[R] How to make try to catch warnings in logistic glm

l)) to indicate if warnings (convergence / probabilities equal to 0/1) occurs (see attempt to code below if my explanation is terse). Best regards, Fredrik Nilsson -- Fredrik Nilsson, PhD Competence Centre for Clinical Researc